1. Field of the Invention
This invention relates to a fuel injection control apparatus for an internal combustion engine of a car, which is concerned with processing measured values of suction air quantity in the internal combustion engine.
2. Background of the Invention
A fuel injection control apparatus for an internal combustion engine of a car of the kind described above is shown in FIG. 1. An electromagnetically driven injector (fuel injection valve) 2 supplies fuel to the internal combustion engine 1. A hot-wire air-flow sensor 3 detects the quantity of air sucked into the engine. A throttle valve 5 provided at a part of a suction pipe 6 regulates the quantity of air sucked into the engine. A water temperature sensor 7 detects the temperature of the engine. A controller 8 computes the quantity of fuel to be supplied to the engine on the basis of an air quantity signal supplied from the air-flow sensor 3 and thereby applies a pulse width corresponding to the required fuel quantity to the injector 2. Further, an igniter 9 generates a pulse signal for the controller 8 at every predetermined rotational angle of the engine. Also shown is a fuel tank 11. A fuel pump 12 applies pressure to the fuel in the tank 11. A fuel pressure regulator 13 maintains constant the pressure of the fuel supplied to the injector 2. Finally, there is shown an exhaust pipe 14. Further, the controller 8 comprises elements 80-84, more specifically an input interface circuit 80, a microprocessor 81 and a ROM 82. The microprocessor 81 is arranged to process various kinds of input signals, to compute the quantity of fuel to be supplied to the suction pipe 6, and from thence to the combustion chamber as determined by the execution of a predetermined program stored in advance in the ROM 82, and to control a drive signal to the injector 2. A RAM 83 temporarily stores data during the execution of computation by the microprocessor 81. An output interface circuit 84 drives the injector 2.
In the operation of the thus arranged conventional engine control apparatus, the quantity of fuel to be supplied to the engine is calculated by the controller 8 on the basis of a suction air quantity signal detected by the air flow sensor 3. At the same time, the rotational frequency of the engine is calculated on the basis of a rotation pulse frequency obtained from the igniter 9, so that a fuel quantity per engine revolution can be calculated. The controller 8 applies a required pulse width to the injector 2 in synchronism with an ignition pulse. The pulse width applied to the injector 2 is corrected so as to be increased or decreased in accordance with a temperature signal generated from the water temperature sensor 7 because it is necessary to set the required air/fuel ratio of the engine to the rich side when the temperature of the engine is low. Further, control is made so as to correct the air/fuel ratio to the rich side by detecting the acceleration of the engine on the basis of a change in the opening of the throttle valve 5.
In the conventional apparatus as described above, however, the hot-wire air-flow sensor 3 used for the fuel control has the desirable characteristic that the provision of new means for correcting atmospheric pressure is not necessary. This arises because the sensor 3 can detect the quantity of suction air by weight. However, the sensor 3 is sensitive to the return blow of air produced by valve overlapping of the engine so that it may detect a signal representing the quantity of suction air in which the quantity of the return-blow air is also included. Accordingly, the output signal generated by the air-flow sensor 3 may express a quantity of suction air which is larger than the actual quantity of the air. Particularly in the low-speed, full-power operation of the engine, return blow is apt to occur. For example, as when in FIG. 2, although the true suction air is not sucked during time t.sub.R, the measured suction air quantity has such a wave form as shown in FIG. 2, which would seem to indicate that the suction air is increased by the return blow. As the result, the output of the air-flow sensor 3 expresses values, as shown in FIG. 3, considerably larger than the true values (shown by broken lines in the drawings), in the low-speed, full-power region. Although varying with the layout of the engine, the suction system, or the like, the error due to the return blow generally reaches about 50% at the maximum so that the sensor 3 cannot be put into practical use as it is.
In order to compensate for such an error, there has been proposed a method in which values for the maximum quantity of suction air (including variations) to be sucked in the engine are set in advance in the ROM 82. As a result as shown in FIG. 4, the output signal a generated from the air-flow sensor 3 is disregarded and clipped to a line of values as shown by "MAX" which are slightly larger (for example, 10%) than an average value b of the true suction air quantity. In this method, however, the clipping values represented by "MAX" imply that the maximum suction air quantity is set for engine operating conditions at sea level and at an ordinary temperature. Accordingly, the air/fuel ratio is greatly shifted to the rich side in the condition of low atmospheric pressure while running at high altitudes or in the case where the temperature of suction air is high, so that there is the possibility of increased fuel cost as well as the possibility of an accidental fire. Further, there is the corresponding problem that the air/fuel ratio is shifted to the lead side where the temperature of the suction air is low.
There has been proposed a method in which wave forms are first determined to be affected by return blow and are then subjected to subtraction to thereby correct a detection error in an air-flow sensor 3 due to such return blow of suction air. However, the waveforms due to the return blow vary depending on both the rotational frequency of the engine and the opening of the throttle valve. Accordingly, it has been impossible to perform accurate correction.
Thus, the conventional fuel injection control apparatus, the problem exists that the hot-wire air-flow sensor 3 detects the suction air quantity as a value larger than the true value thereof because of the return blow of air produced in low-speed, full-power operation, so that the air/fuel ratio cannot be controlled appropriately in a certain running region.